1. Field of the Invention
The present invention relates to a system for accurate placement of collection optics in microscopes.
2. Description of the Related Art
Various microscopes are used in the art for imaging, testing, and examination of various microstructures. A common feature of these microscopes is that the obtained resolution depends on efficient collection of light from the inspected object. Accurate placing of the collection optics is also highly important for accurate focusing on the object to be inspected. Additionally, to obtain efficient collection of light, the collection optics needs to be accurately placed with respect to the object to be imaged.
While collection efficiency is highly important for many types of microscopes, it is imperative in one particular field: probing and testing of semiconductor microchips. Microchips need to be tested during the design and during the manufacturing stages. One type of testing relies on light emission from the microchip that is generated whenever a device, e.g., a transistor, on the microchip changes state. For further information on this phenomenon and it's investigation, the reader is directed to, for example:
All-Solid-State Microscope-Based System for Picosecond Time-Resolved Photoluminescence Measurements on II-VI semiconductors, G. S. Buller et al., Rev. Sci. Instrum. pp. 2994, 63, (5), (1992);
Time-Resolved Photoluminescence Measurements in InGaAs/InP Multiple-Quantum-Well Structures at 1.3 μm Wavelengths by Use of Germanium Single-Photon Avalanche Photodiodes, G. S. Buller et al., Applied Optics, Vol 35 No. 6, (1996);
Analysis of Product Hot Electron Problems by Gated Emission Microscope, Khurana et al., IEEE/IRPS (1986);
Ultrafast Microchannel Plate Photomultiplier, H. Kume et al., Appl. Optics, Vol 27, No. 6, 15 (1988); and
Two-Dimensional Time-Resolved Imaging with 100-ps Resolution Using a Resistive Anode Photomultiplier Tube, S. Charboneau, et al., Rev. Sci. Instrum. 63 (11), (1992).
Notably, Khurana et al., demonstrated that photoluminescence emission coincides with the switching of a transistor, thereby showing that, in addition to failure analysis, the phenomenon can also be used for device debug and circuit design. See, also, U.S. Pat. No. 5,940,545 to Kash et al., disclosing a system for such an investigation.
As can be appreciated from the above-cited works, the light emission in semiconductor devices is very faint. Accordingly, various optical and detection schemes have been proposed to more efficiently collect the emission and reduce the noise, i.e., increase collection fidelity, bandwidth, and speed. For example, commercially available microchannel photomultipliers have been used to amplify the collected light by factors of a million or so. Also, avalanche diodes coupled to time- to-amplitude converters (TAC) have been used to provide high temporal resolution of the emission.
From the optics perspective, various attempts have been made to increase the collection of light and the resolution by increasing the numerical aperture (NA n*sin θ; n being the index of refraction and θ being the collection angle) of the objective lens. It has been long known that increasing the numerical aperture can be achieved by increasing the index of refraction, n, to be above that of air. One historical method for increasing n is to fill the gap between the objective lens and the object with an index matching oil. Another method is to use an immersion lens between the object and the objective lens. Of course, one may use both techniques, i.e., use immersion lens and index matching fluid. The use of the above techniques is disclosed in, for example, U.S. Pat. Nos. 3,524,694, 3,711,186, and 3,912,378. More modern discussions of immersion lenses can be found in U.S. Pat. Nos. 4,634,234, 5,004,307, 5,208,648, 5,282,088 and Solid Immersion Microscopy, S. M. Mansfield, G. L. Report No. 4949, Stanford University 1992. Prior art immersion lenses are plano-convex (i.e., hemispheres). That is, the bottom surface, i.e., the surface facing the object, is flat, while the top surface, i.e., the surface facing the objective lens, is convex.
A semiconductor device of particular interest to the present invention is generally referred to as a “flip chip.” Since the structure of flip chips is known, it will not be discussed here in detail. Information relating to flip chips can be found in, for example, http://www.flipchip.com and http://world.std.com/˜hycomp/flippage.html. Of specific interest is recent effort in the art to inspect such devices from the back side, i.e., from the substrate side. One problem in testing flip chips using conventional methods, such as e-beam testing, is that the metal lines are not readily accessible as in other integrated circuits. Therefore, in order to expose the metal lines to the e-beam tester, one needs to employ a forced ion beam (FIB) to “drill” through the substrate and expose the metal lines. However, with the density of today's IC's, the active devices occupy much of the “real estate” available on the substrate, thereby rendering the use of FIB impossible. Therefore, the mechanism of light emission described above has been employed also for probing flip chips from the back side. The reader is directed to these three articles, published in the Proceedings of 1998 International Test Conference (ITC 98), Oct. 18-22, 1998, Washington, D.C., IEEE Catalog No. RSOO191:
Novel Optical Probing Technique for Flip Chip Packaged Microprocessors, Mario Paniccia, Travis Eiles, V. R. M. Rao and Wai Mun Yee.
Diagnosis and Characterization of Timing-Related Defects by Time-Dependent Light Emission, Dave Vallett, Leendert Huisman, and Phil Nigh.
Contactless Gigahertz Testing, W. Mertin, A. Leyk, U. Behrike. and V. Wittpahl.
Another article of interest is Picosecond Noninvasive Optical Detection of Internal Electrical Signals in Flip-Chip-Mounted Silicon Integrated Circuits, H. K. Heinrich, IBM J. Res, Develop. Vol 34, No. 2/3 1990.
Systems for imaging flip-chips from the backside through the silicon substrate are described in U.S. Pat. Nos. 5,208,648, 5,220,403 and 5,940,545.
However, in spite of the amount of work in the field, there is still no commercially viable system for device debug by time resolved measurements of hot electron emission, as opposed to device illumination. For example, one of the issues causing difficulties is the efficient collection of the faint light emission. Such an efficient collection requires highly accurate control and placement of the collection optics.